This invention relates to foams of ethylene vinyl acetate. This invention specifically relates to microcellular foams of ethylene vinyl acetate and ethylene/acrylic acid copolymer as well as improved methods of production.
Ethylene and vinyl acetate (EVA) copolymers are well known items of commerce having a wide variety of applications. Ethylene vinyl acetate polymers are produced by copolymerizing ethylene and vinyl acetate monomer. As the bulky acetoxy group content increases, the polymer becomes more amorphous, and possesses increased flexibility, rubberiness, low temperature properties, tackiness and heat sealability. This thermoplastic material is therefore widely used in flexible packaging, hot melt adhesives, electrical, medical and many other applications.
EVA resins can be conveniently crosslinked by both peroxide or irradiation to enhance mechanical properties and heat resistance. It is this crosslinking, coupled with the inherent rubbery nature of the polymer which makes EVA suitable for production of tough and abrasion resistant foams, especially suitable for footwear applications.
Crosslinked EVA foams can be manufactured by two methodsxe2x80x94the ionizing radiation method and the chemical crosslinking method. The ionizing method, however, is restricted to pieces less than xc2xc inches thick, and thus is of limited use. The chemical crosslinking method has found more commercial applicability. By crosslinking, the viscosity of EVA at high temperatures is increased and the individual cell is kept in a stable condition without rupture or agglomeration. Low-density microcellular foam can be obtained. By selecting the vinyl acetate (VA) content, the EVA foam is flexible and highly resilient with easy coloring and adherent to other materials. The application is used widely in shoe soles, sandals and cushion materials.
The prevalent chemical crosslinking method for producing microcellular cross-linked EVA foams is the compression molding process. Although this process produces high quality foam, it requires long processing time and generates a high level of scrap. A press-molding foam process involves many steps: compounding, press molding into foam, cutting into shape, then press molding into a final product. The process is long, tedious and labor intensive. Improving productivity would be highly desirable, as well as improving foam performance to achieve lighter density and improved properties.
The injection molding process was developed more than ten years ago to overcome the drawbacks of compression molding. This process has not been widely accepted due to the lack of high quality consistent compounds to reproduce exactly the same size from shot to shot over production cyles lasting several days. In such processes, achieving a balance between foam density and performance properties (e.g. compression set) can be difficult to attain. See, e.g., xe2x80x9cMicrocellular Crosslinked EVA Foam By Injection Molding Processxe2x80x9d, John Lee, 2060/ANTEC, 97.
There is a continued need to improve EVA foam properties, especially to improve compression set resistance for lower density EVA foams. A need also exists to develop EVA compounds that can enable shorter, more cost-effective processes to produce EVA foam.